This invention relates to catalysts suitable for catalytic aromatization of olefins and paraffins the catalysts comprising ZSM-5 or ZSM-11, to a method of making such catalysts and to an aromatization process using same.
Aromatic hydrocarbons are important commodity chemicals in the petroleum industry. The most commercially valuable aromatics are the BTX compounds (benzene, toluene, ethylbenzene and xylenes). Aromatics are currently produced by catalytic cracking of naphthas, catalytic reforming of various petroleum feedstocks etc. They can also be produced by catalytic conversion of alcohols (particularly methanol) or olefins. The catalysts used in these processes (methanol-to-gasoline or MTG, olefins-to-gasoline-and-distillate, or MOGD or M2 Forming, both developed by Mobil Oil) belong to the pentasil zeolite family whose most important member in terms of industrial applications is the ZSM-5 structure. The latter is a tridimensional crystalline aluminosilicate having strong acid sites and whose intermediate pore (or channel) system displays a reaction shape selectivity which leads to the production of substantial amounts of monoaromatics.
In the M2 Forming and MOGD processes, the ZSM5 in the acid, or hydrogen, form is used alone without any cocatalyst which would promote the aromatizing action of the catalyst component itself.
Zinc oxide and alumina are known as catalysts or modifiers of zeolite catalysts for the purpose of hydrocarbon aromatization. A number of references discloses the use of various metal cations to modify the zeolite pore diameter to favor the formation of the para-isomer of dialkylaromatics.
U.S. Pat. No. 3,325,552 issued to Wennerberg discloses an aromatizing catalyst comprising three different metal oxides: zinc oxide, magnesia and alumina.
U.S Pat. No. 4,036,902 (Gregory) teaches an aromatization catalyst comprising alumina promoted by zinc or a zinc compound.
U.S. Pat. No. 4,590,322 (Chu) discloses a process for producing aromatic hydrocarbons using a catalyst comprising a zeolite such as ZSM-5 which contains optionally a difficultly reducible oxide incorporated into the zeolite to enhance the para-selectivity properties of the catalyst modified therewith. This patent, as well as U.S. Pat. Nos. 4,590,321 and 4,590,323 issued to the same inventor, teach that zeolites may be composited with a porous matrix material such as alumina, and with some binary or ternary oxide compositions in the form of a cogel. The cogel, which may include zinc oxide and/or aluminum oxide is incorporated into the structural lattice of zeolite particles.
Zinc is also an aromatization promoter when incorporated into the zeolite lattice by ion exchange or by impregnation as taught in Chang et al. U.S. Pat. No. 3,894,104. However, zinc and other modifying ions/metals, according to the Chang patent, must be in close contact with the zeolite acid sites in such a way as to be able to effect the promoting action on the aromatizing acid sites over very short distances, not exceeding a few Angstroms.
Moreover, it is known to obtain certain catalysts by mixing the components physically. Canadian Patent No. 1,204,099 (Heyward) discloses a catalytic composition for conversion of synthesis gas to hydrocarbons, the composition comprising a mixture of a tectometallosilicate, e.g. a zeolite, zinc oxide, an oxide of gallium or indium and an oxide of an additional metal selected from the elements of Group 1B, lll-V, VlB, VllB and Vlll.
U.S. Pat. No. 4,538,017 (Butler) uses a catalyst composed of silicalite and zinc oxide for the aromatization of paraffinic feedstocks.